/* Copyright (C) 2000-2019 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdint.h>
//#include <sys/fcntl.h>
#include <sys/stat.h>
#include <sys/statfs.h>

/* Reserve storage for the data of the file associated with FD.  This
   emulation is far from perfect, but the kernel cannot do not much
   better for network file systems, either.  */

int
posix_fallocate (int fd, off_t offset, off_t len)
{
    struct stat st;

    if (offset < 0 || len < 0)
        return EINVAL;

    /* Perform overflow check.  The outer cast relies on a GCC
       extension.  */
    if ((off_t) ((uint64_t) offset + (uint64_t) len) < 0)
        return EFBIG;

    /* pwrite below will not do the right thing in O_APPEND mode.  */
    {
        int flags = fcntl (fd, F_GETFL, 0);
        if (flags < 0 || (flags & O_APPEND) != 0)
            return EBADF;
    }

    /* We have to make sure that this is really a regular file.  */
    if (fstat (fd, &st) != 0)
        return EBADF;
    if (S_ISFIFO (st.st_mode))
        return ESPIPE;
    if (! S_ISREG (st.st_mode))
        return ENODEV;

    if (len == 0)
    {
        /* This is racy, but there is no good way to satisfy a
	       zero-length allocation request.  */
        if (st.st_size < offset)
	    {
	        int ret = ftruncate (fd, offset);

	        if (ret != 0)
	            ret = errno;
	        return ret;
	    }
        return 0;
    }

    /* Minimize data transfer for network file systems, by issuing
       single-byte write requests spaced by the file system block size.
       (Most local file systems have fallocate support, so this fallback
       code is not used there.)  */

    unsigned increment;
    {
        struct statfs f;

        if (fstatfs (fd, &f) != 0)
            return errno;
        if (f.f_bsize == 0)
            increment = 512;
        else if (f.f_bsize < 4096)
            increment = f.f_bsize;
        else
            /* NFS does not propagate the block size of the underlying
	           storage and may report a much larger value which would still
	           leave holes after the loop below, so we cap the increment at
	           4096.  */
            increment = 4096;
    }

    /* Write a null byte to every block.  This is racy; we currently
       lack a better option.  Compare-and-swap against a file mapping
       might additional local races, but requires interposition of a
       signal handler to catch SIGBUS.  */
    for (offset += (len - 1) % increment; len > 0; offset += increment)
    {
        len -= increment;

        if (offset < st.st_size)
	    {
	        unsigned char c;
	        ssize_t rsize = pread (fd, &c, 1, offset);

	        if (rsize < 0)
	            return errno;
	        /* If there is a non-zero byte, the block must have been
	           allocated already.  */
	        else if (rsize == 1 && c != 0)
	            continue;
	    }

        if (pwrite (fd, "", 1, offset) != 1)
	        return errno;
    }

    return 0;
}

